Bug 44173 – Scrolling is slow and stuttery on some long pages (due to accelerated compositing?)

• Component: Layout and Rendering
• Status: RESOLVED INVALID

[Adam Roben (:aroben)]

To reproduce:

1. Open the attached Web Archive
2. Drag the scrollbar down to the bottom

When you reach about 2/3 of the way down the page (in the middle of the "Storport ETW Logging to Measure Requests Made to a Disk Unit" article), scrolling becomes slow and stutters. The scroll thumb visibly lags behind the mouse.

I've only tested with Safari 5.0.1 on SnowLeopard.

[Adam Roben (:aroben)]

Created attachment 64701 [details]
testcase

[Adam Roben (:aroben)]

<rdar://problem/8324086>

[Adam Roben (:aroben)]

Here's an even easier to reproduce case:

1. Go to http://trac.webkit.org/browser/trunk/WebKit/win/WebView.cpp?rev=58900#L946
2. Drag the scroll bar thumb

Scrolling is horribly slow. Tiles of the page visibly repaint as you go.

Turning off accelerated compositing brings performance back to acceptable levels.

[Adam Roben (:aroben)]

Whoops, I had an extension installed that turned on accelerated compositing for all <img> elements. Without that extension these pages don't go into accelerated compositing mode. Simon Fraser assures me that we already have bugs that cover the symptoms I was seeing on pages that legitimately use accelerated compositing, so I'm going to close this bug.